Ok, since you asked.
On a road bike, any kind, as long as you are riding on a road, the rear brake is technically useless. In an emergency situation, you can grab as much rear brake as possible but the rear wheel will begin to skid at a fairly low rate of deceleration. When you apply your front brake to it's maximum potential, (without the use of a rear brake) you will come to a stop much much faster because it is impossible to slide the wheel. Combine the transition of force directly to the tire and the fact that the coefficient of static friction is greater than that of kinetic friction, by using your front brake only you will come to a stop in about half the time you could by using only your back brake.
Now that you understand that, the reason the back brake is useless is that when you are using your front brake to it's full potential, all of your weight has shifted to the front tire and the rear tire has essentially been unweighted. At this point, ANY application of rear brake will simply cause the rear wheel to lock up and skid. Any amount of braking less than in an emergency situation can be handled easily with only the front brake as well.